std::locale::operator()

From Cppreference

Compares two string arguments s1 and s2 according to the lexicographic comparison rules defined by this locale's std::collate<charT> facet. This operator allows any locale object that has a collate facet to be used as a binary predicate in the standard algorithms (such as std::sort) and ordered containers (such as std::set)

[edit] Parameters

[edit] Return value

true if s1 is lexicographically less than s2, false otherwise.

[edit] Equivalent function

template<class CharT, class Traits, class Alloc >
bool operator()(const std::basic_string<CharT,Traits,Alloc>& s1,
                const std::basic_string<charT,Traits,Alloc>& s2) const;
{
    return std::use_facet<std::collate<charT>>(*this).compare(
                                         s1.data(), s1.data() + s1.size(),
                                         s2.data(), s2.data() + s2.size()   ) < 0;
}

[edit] Example

A vector of strings can be sorted according to a non-default locale by using the locale object as comparator:

#include <locale>
#include <algorithm>
#include <vector>
#include <string>
#include <cassert>
 
int main()
{
    std::vector<std::wstring> v = {L"жил", L"был", L"кот"};
    std::sort(v.begin(), v.end(), std::locale("ru_RU.UTF8"));
    assert(v[0] == L"был");
    assert(v[1] == L"жил");
    assert(v[2] == L"кот");
}

[edit] See also